Hitherto, there is proposed an image display apparatus including a spatial light modulation element and adapted for projecting, by projection lens, an image of the spatial light modulation element onto screen, etc. to perform image display. As image display apparatuses of this kind, there is an apparatus having a configuration as shown in FIG. 1. The image display apparatus shown in FIG. 1 comprises an illumination optical system 102 including a light source 101 such as discharge lamp, etc., and serves to illuminate, by the illumination optical system 102, a spatial light modulation element 103 using polarization such as liquid crystal, etc. to project, by a projection lens 104, an image of the spatial light modulation element 103 onto screen (not shown). The image display apparatus of the projection type has been put into practical use as large-sized image display apparatus.
As a spatial light modulation element used in the image display apparatus of this kind, there is used a spatial light modulation element of the reflection type including reflection electrode. In such spatial light modulation element of the reflection type, aperture ratio can be enlarged, and miniaturization and high definition (image) can be realized.
The image display apparatus shown in FIG. 1 generally uses polarization beam splitter (PBS) 105 as polarizer and analyzer. Namely, light beams emitted from the illumination optical system 102 are incident on the polarization beam splitter 105 serving as polarizer so that only component in specific polarization direction is selected. The light beams thus obtained are incident on the spatial light modulation element 103.
Between the polarization beam splitter 105 and the spatial light modulation element 103, there is disposed a dichroic prism 106 as color separation/analysis element. Namely, illumination light which has been passed through the polarization beam splitter 105 is color-separated into R (red), G (green) and B (blue) at the dichroic prism 106. The respective color components are incident on corresponding to spatial light modulation elements 103, 103, 103. These incident color components are polarized and modulated. The color components thus obtained are reflected.
Reflected light beams from the respective spatial light modulation elements 103, 103, 103 corresponding to respective color components of R, G, B are color-synthesized at the dichroic prism 106. The light beams thus obtained are incident on the polarization beam splitter 105. Here, the polarization beam splitter 105 acts (functions) as analyzer to allow only specific polarized light component to be passed therethrough to thereby convert polarization modulation at the respective spatial light modulation elements 103, 103, 103 into intensity modulation. As the result of the fact that light beams which have been intensity-modulated in this way are incident on the projection lens 104, images corresponding to modulations at the respective spatial light modulation elements 103, 103, 103 are projected and displayed on the screen.
In the image display apparatus as described above, polarization beam splitter used as polarizer and analyzer serves to select polarized light component by difference between reflection factor of P-polarized light and reflection factor of S-polarized light at face of dielectric multi-layer film, and has great wavelength dependency and great angle dependency of incident light. For this reason, in this image display apparatus, bright illumination optical system having low F number cannot be used. As a result, it is difficult to improve light utilization efficiency.
In the above-described image display apparatus, dichroic prism used as color separation/synthesis element has large polarization dependency. Namely, characteristics at dichroic surfaces of incident light (S-polarized light) and outgoing light (P-polarized light) are different from each other so that polarization axis of modulated light which has been passed through the spatial light modulation element is placed in the direction perpendicular to incident light polarization axis. For this reason, light utilization efficiency is lowered.
Here, there is conceivable an optical system in which the optical axis of the illumination optical system is included (tilted) with respect to the spatial light modulation element to thereby use polarization plate in place of polarization beam splitter. In this optical system, in order that eclipse (shading) of illumination light by mirror cylinder of projection lens does not take place, it is necessary to increase tilt (inclination) with respect to the spatial light modulation element of the optical axis of the illumination optical system. When the optical axis of the illumination optical system is greatly tilted with respect to the spatial light modulation element, the optical axis of light beams incident on the projection lens is also tilted in accordance with this tilt (inclination), the characteristic of the projection lens is deteriorated. Further, incident angle of illumination light onto polarization plate or spatial light modulation element is increased so that illumination efficiency is lowered, and contrast of display image is lowered.